The presence of salts allows seawater to conduct electricity by electrolytic ion exchange. By passing an electric current through seawater in the presence of a magnetic field, a Lorentz (J.times.B) force will act to move the seawater in a direction normal to both the magnetic field and electric current directions. This is the basis for magnetohydrodynamic (MHD) propulsion. Few mechanical moving parts are required with MHD propulsion, and as a result, MHD propulsion can be very quiet. MHD technology became practical with the advent of multi-Tesla superconducting magnets. With superconducting magnets, the magnetic field can be increased while the magnet's weight and electrical consumption remain relatively low.
Recently there has been an increase of research and development activity in MHD propulsion. An experimental ship, YAMATO-1, utilizing MHD propulsion has recently been built and tested in Japan. Research in the United States includes thruster experiments at the Argonne National Laboratory (ANL), the Naval Undersea Warfare Center-Newport (NUWC-N) and the Applied Research Laboratory of Penn State University (ARL-Penn State). Analytical studies of seawater propulsion and seawater electrolysis, conductivity enhancements, and electrode studies have also been conducted. The experiments of ANL, NUWC-N, and ARL-Penn State use 6-, 3.2- and 8-Tesla (T) magnets, respectively. Among them, the magnets for ANL and NUWC are superconducting dipoles, while the one for ARL-Penn State is an electrosolenoid.
For descriptions of the aforesaid efforts, see: Motora, et al. (1991), "An Outline of the Superconducting MHD Ship Propulsion in Japan", Proc. MHDS91, Int. Symp. on Superconducting Magnetohydrodynamic Ship Propulsion, Paper No. 1-1; Petrick et al., (1991), "Magnetohydrodynamic Seawater Propulsion", Proc. MHDS91, Int. Symp. on Superconducting Magnetohydrodynamic Ship Propulsion, Paper No. 5-2; Meng et al., (1991), "Experimental Studies of Superconducting electromagnetic Thruster for Seawater Propulsion and Future Technology Challenges", Proc. MHDS91, Int. Symp. on Superconducting Magnetohydrodynamic Ship Propulsion, Paper No. 5-4; Lin et al., (1992), "Study of the Influence of Electric and Magnetic Fields on Seawater Magnetohydrodynamic Propulsion", Proc 2nd International Offshore and Polar Engg Conf., Vol. III pp. 8-13; Doss et al. (1991), "Flow Characteristics Inside MHD Seawater Thrusters", J. Propulsion and Power, Vol. 7, No. 4, pp. 635-641; Lin et al., (1991), "Analyses of Magneto-Hydrodynamic Propulsion with Seawater for Underwater Vehicles", J. Propulsion and Power, Vol. 7, No. 6, pp. 1081-1083; Lin (1990) "Consideration of Seawater Conductivity Enhancements for Electromagnetic Thrusters", Proc. 25th Inter-Society Energy Conversion Engr. Conf., Vol. 5, pp. 552-556; and Gilbert et al. (1991), "Seawater Conductivity Enhancement by Acid Seeding for Magnetohydrodynamic Propulsion", AIAA Paper No. AIAA-91-2499, 27th Joint Propulsion Conf., Sacramento, Calif., June 24-26.
The aforementioned studies have concentrated on straight duct MHD channels which use a long bore, and a low magnetic field dipole magnet (.ltoreq.6T). Unfortunately, a dipole magnet requires extensive structural support which limits its attainable magnetic field. Much higher fields can be achieved with a large diameter bore solenoid magnet due to its axial-symmetric winding. However, straight MHD thrusters, when used in solenoid magnets, have limited thrust due to small aspect ratios.
Taking advantage of the high-field characteristics of the solenoid, single- and double-loop cyclotron systems were investigated (i.e. see: Aumiller et al. (1993), "Analytical and Experimental Studies of the Cyclic Magnetohydrodynamic Thruster Designs", Proc. 3rd International Offshore and Polar Engg. Conf. Vol. I, pp. 46-53). In the cyclotron design, the magnetic field and electric current are applied axially and radially, respectively. As a result, an MHD flow is induced in the azimuthal direction allowing increased active length and better utilization of the higher magnetic fields near the windings. Results from these studies were encouraging with the double-loop system exhibiting a higher efficiency than the single-loop.
There is a continuing need for quiet submarine propulsion units. Further, there is a need for quiet maneuvering jets for use in submarine devices. However, previous MHD propulsion units have not exhibited sufficient levels of efficiency to make them practical for such applications.
Accordingly, it is an object of this invention to provide an improved MHD propulsion unit which employs a solenoid coil structure.
It is another object of this invention to provide am improved MHD unit which can be used either for propulsion purposes or electrical generation purposes.